Chorionic villus sampling (CVS) is an established method for prenatal diagnosis of genetic diseases and birth defects in the first trimester of pregnancy, which has been in use since the 1980s. The procedure entails retrieving a sample of the chorionic villus or placental tissue and testing it. CVS is an alternative to genetic amniocentesis, a procedure used since the 1960s for essentially the same purpose, and may be performed sooner. Amniocentesis typically involves inserting a thin needle through the patient's abdominal wall through the wall of the uterus into the amniotic sac under ultrasound guidance during the 16-20 week of pregnancy. The CVS procedure may be carried out 10-13 weeks after the patient's last menstrual period. In the event that the fetus is found to be affected with a serious abnormality, earlier diagnosis allows for safer, and less emotionally traumatic, termination of the pregnancy due to the development of the fetus over time.
Chorionic villi are frond-like projections from the outer membrane or chorion of the anmiotic sac. These projections eventually form the placenta. Because chorionic villi are derived from the same fertilized egg as the fetus, they can be used to determine the genetic health of the fetus. The CVS procedure is done either by inserting a thin catheter, consisting of a cannula containing an obturator, under ultrasound guidance through the vagina and cervix into the chorionic villi of the chorion frondosum (transcervically), or by insertion of a thin needle through the abdominal wall (transabdominally). Once the cannula tip is placed within the chorion frondosum and the obturator is withdrawn, negative pressure is applied by some means, such as a syringe, to aspirate a sample of chorionic villi. Both methods of the CVS procedure may be done in a matter of minutes.
Biopsy catheters for use in the CVS procedure have heretofore been disclosed or manufactured in a number of ways. U.S. Pat. No. 5,106,377 to Martin describes various types. One such existing catheter consists of a hollow flexible cannula and a 1.5 millimeter diameter flexible aluminum obturator, which fits snugly in the cannula to facilitate inserting the cannula into position. When in use, the physician inserts the obturator into the cannula and then bends the resulting catheter to obtain the desired degree of bend for insertion into the patient. After the resulting catheter is maneuvered into the correct position, the obturator is withdrawn. The Martin patent mentions that a drawback of this catheter is the aluminum obturator tends to flex the curved portion of the cannula when removed, thus deflecting the cannula tip from the desired location.
Another catheter disclosed in the Martin patent consists of a malleable silver cannula that contains a flexible blunt stainless-steel obturator during insertion. The obturator supports the cannula to permit the cannula to be flexed without kinking thereby ensuring the tubular cannula's internal continuity. Likewise, with this catheter the stiffness of the obturator causes deflection of the cannula tip during withdrawal of the obturator. Additionally, the stiffness of the catheter would often cause ruptures in patients' membranes.
Yet another catheter disclosed in the Martin patent uses an aluminum obturator in a flexible plastic cannula that may be bent to the desired degree of curvature. Once again, when the obturator is removed its relative stiffness will cause flexing in the cannula and movement of the tip. Therefore, it has been previously disclosed that biopsy catheters for use in the CVS procedure, which use an obturator having greater stiffness than the cannula, may suffer from the disadvantage of tip displacement when the obturator is withdrawn.
U.S. Pat. No. 4,756,708, also to Martin, purportedly overcame this disadvantage with a preformed curved cannula and a very flexible obturator, which did not cause deflection of the cannula tip upon removal. However, this catheter was found not to work in all cases because the preset curvature angle did not fit the anatomy of some patients. Martin sought to remedy this with the catheter disclosed in U.S. Pat. No. 5,106,377. The catheter disclosed therein consists of an obturator of a synthetic plastic material such as polyurethane. The cannula consists of an outer sleeve of Nylon with an inner tube of a thin wall stainless steel. After inserting the obturator into the cannula the catheter is bent to the desired angle of curvature before maneuvering the catheter into position to collect the chorionic villi sample. The stiffer cannula construction was to prevent deflection of the tip when the obturator is removed. The stiffness of this catheter, however, was found by some practitioners to be undesirable.
Another form of CVS catheter is produced by Cook Urological in Spencer, Ind. This product, which is designated the Cook Chorionic Villus Sampling Set, consists of a hollow cannula with a malleable metal obturator. The catheter is manufactured in 1.9 millimeter and 1.8 millimeter diameters. The 1.9 millimeter size is available in 21 centimeter and 27 centimeter lengths. The 1.8 millimeter catheter is 24 centimeters in length. The obturator for each of these catheters includes a flat molded plastic handle. Because the handle is flat, it is difficult to manipulate between the fingers of the physician/operator when inserting the catheter to the desired location to collect the sample. The difficulty in manipulating this flat handle has been found to make placement of the catheter less precise in certain anatomical positions of the pregnant uterus.